1. Field of the Invention
The present invention relates to an ink composition for organic electroluminescent (EL) devices, more particularly to an ink composition for forming an emitting layer of organic EL devices by coating, and to a production method thereof.
2. Description of the Related Art
Organic EL devices can be broadly classified into high-molecular type and low-molecular type according to the material of their emitting layer. While low-molecular organic emitting layers are often formed by vapor deposition, high-molecular or polymer organic emitting layers are often formed by application of inks containing polymer organic EL materials (see Japanese Patent Application Laid-Open No. 10-012377, for example).
Polymer organic EL materials prepared by palladium- or nickel-catalyzed coupling of aromatic compounds are known (see Japanese Patent Application Laid-Open No. 2004-115587, for example). As available coupling reactions, for example, Suzuki coupling reaction and Yamamoto coupling reaction are known. Moreover, polymer organic EL materials prepared by polymerization of aryl magnesium halides by Grignard reaction are also known (see Japanese Patent Application Laid-Open No. 2003-238666, for example).
Organic EL devices with an emitting layer composed of the above polymer organic EL material, however, have met with problems of reduced brightness during initial operation, shorter lifetime, etc., under certain circumstances.
It is therefore required for polymer organic EL materials to prevent brightness reduction that occurs during initial operation of organic EL devices and to increase their lifetime; however, this objective has not yet been achieved. In view of this situation it is an object of the present invention to provide an emitting material for long-life, polymer organic EL devices in which brightness reduction during initial operation is small.
The inventors investigated the cause of the brightness reduction and shorter lifetime of an organic EL device having an emitting layer composed of polymer organic EL material. As a result, they discovered that impurities in polymer organic EL materials, such as metals, halogens and low-molecular organic components, are responsible for the reduction in the brightness and lifetime. In particular, in the case where an organic EL device includes an emitting layer composed of polymer organic EL material containing alkoxy groups at the polymer chain terminals, they discovered that alkoxy group-containing active species are liberated upon exposure to an electric field and reduce brightness and lifetime. Based on these discoveries they established that the foregoing problems can be overcome by previously releasing the impurities in the ink composition and removing them.
Specifically, the foregoing problems can be solved by the following means.
[1] A production method of an ink composition for organic EL devices including:
preparing a composition containing a polymer organic EL material and an organic solvent; and
applying an electric field to the composition.
[2] The method according to [1], wherein the composition contains: a polymer organic EL material prepared by a coupling reaction between a halogenated aromatic compound and an aromatic boron compound in the presence of palladium catalyst or nickel catalyst; an organic solvent; and an aromatic carboxylic acid, and wherein the aromatic carboxylic acid is contained in an amount of 0.01-1 wt % based on the total amount of the organic solvent and the aromatic carboxylic acid.[3] The method according to [1] or [2], wherein the intensity of the electric field is 3-10 kV/mm.[4] The method according to any one of [1] to [3], wherein the polymer organic EL material is a polymer material containing a repeating unit having the following General Formula (a1).
where R denotes a hydrogen atom or alkyl group having 1-4 carbon atoms, and R1 denotes a hydrogen atom, alkyl group having 1-4 carbon atoms, aryl group, or alkylaryl group.[5] The method according to any one of [2] to [4], wherein the aromatic carboxylic acid is a compound having the following General formula (c10) or General Formula (c11).
where R10s independently denote a hydrogen atom, alkyl group having 1-12 carbon atoms, alkoxy group having 1-12 carbon atoms, alkoxyalkyl group having 1-12 carbon atoms, or 5- to 7-membered heterocyclic ring.
where Ar10 denotes an aryl group or alkylaryl group, and X denotes a single bond, alkylene group having 1-3 carbon atoms, or —CH2—O—CH2—.[6] The method according to anyone of [2] to [5], wherein the aromatic carboxylic acid has a boiling point of 200-270° C.[7] An ink composition for organic EL devices including:
a polymer organic EL material prepared by a coupling reaction between a halogenated aromatic compound and an aromatic boron compound in the presence of palladium catalyst or nickel catalyst;
an organic solvent;
an aromatic carboxylic acid; and
an ester derivative of the aromatic carboxylic acid,
wherein the aromatic carboxylic acid and the ester derivative are contained in an amount of 0.01-1 wt % based on the total amount of the organic solvent, the aromatic carboxylic acid and the ester derivative.
[8] A manufacturing method of an organic EL device including:
applying an ink composition containing a polymer organic EL material on a pixel electrode or either a hole injection/transport layer or an intermediate layer deposited on the pixel electrode; and
drying the ink composition applied,
wherein the ink composition is an ink composition for organic EL devices which is produced by the method according to any one of [1] to [6].
[9] A manufacturing method of an organic EL device including:
applying the ink composition according to [7] on a pixel electrode or either a hole injection/transport layer or an intermediate layer deposited on the pixel electrode; and
drying the ink composition applied.
The present invention can provide a long-life polymer organic EL material in which brightness reduction during initial operation is small.